Method and means for operating x-ray tubes with rotary anodes

ABSTRACT

In X-ray tubes anodes are used which, are maintained in rotation during the production of X-rays for fluoroscopy and which anodes are supported by friction-diminishing members. The anodes are maintained in rotation for all operational periods during which there may be a wish to take X-ray radiographs. The anode is supported magnetically without contact, except an electrical one for transmitting current to the anode.

United States Patent Seifert et al.

METHOD AND MEANS FOR OPERATING X-RAY TUBES WITH ROTARY ANODES Inventors: Gerd Seiiert; Giinther Appelt, both of Erlangen, Germany Assignee: Siemens Aktiengesellschaft, Munich,

Germany Filed: Dec. 11, 1973 Appl. No.: 423,665

Foreign Application Priority Data Dec. 21, 1972 Germany 2262757 US. Cl. 250/402; 250/406; 313/60 Int. Cl. G03b 41/16 Field of Search 250/402, 520, 406; 3 13/60 References Cited UNITED STATES PATENTS 3/1970 Takano 313/60 [451 Apr. 15, 1975 Primary Examiner-James W. Lawrence Assistant ExaminerC. E. Church Attorney, Agent, or Firm-.-Ri'chards & Geier [57] ABSTRACT In X-ray tubes anodes are used which, are maintained in rotation during the production of X-rays for fluoroscopy and which anodes are supported by'frictiondiminishing members. The anodes are maintained in rotation for all operational periods during which there may be a wish to take X-ray radiographs. The anode is supported magnetically without contact, except an electrical one for transmitting current to the anode.

15 Claims, 4 Drawing Figures METHOD AND MEANS FOR OPERATING X-RAY TUBES WITH ROTARY ANODES This invention relates to method and means for operating X-ray tubes with rotary anodes, the anode of which is maintained in rotation during X-ray fluoroscopy and is supported by friction-diminishing members. I

In using X-ray tubes with rotary anodes it is disantvageous that prior to starting the production of X-rays, one has to wait until the anode is brought to rotation. This is a loss of time which lasts as a rule, up to some seconds. For that reason, expensive starting devices were provided to shorten the starting time period. On the other hand, due to the wear of usual bearings, which is the most serious limit of the lifetime of rotary X-ray tubes, it is not possible to let the anode turn continuously longer than is necessary for specific radiographing situations. This is particularly the case when it is desired to increase the load of the anode possible during an extremely quick turningof the anode. For such arrangements, additionally braking devices have been provided in order, on the one hand, to diminish the bearing wear which increases with the square of the rotary speed, and, on the other hand, to diminish disturbing sounds which increase with the rotary speed.

The present invention is based on the consideration that when X-ray tubes with rotary anodes are operated, it is desirable to make spontaneous photographs without having to take into consideration disturbing or detrimental factors, such as, for example, the starting time, noises and wear of the bearings.

Consequently, an object of the present invention is the provision of a method by the use of which, within the usual operational conditions, it is possible to have X-ray photographing devices with rotary anodes which are ready for photographing at any time, while providing sufficient stability for the tube.

A process of the present invention for operating X-ray tubes with rotary anodes and accomplishing this object of the present invention consists in that the anode is caused to rotate and continues in rotation when and so long as there .is a possibility of an X-ray photograph being taken and that the anode is magnetically supported without contact, with the exception of a preferably axial bearing transmitting the tube current.

The present invention provides a rotary anode that can run for long working periods, namely, for an entire working day or even longer, without the bearing wearing out. Besides elimination of noises, the tube is held in continuous 'working readiness for photographing. Photographs can also be made at unpredetermined moments, which can be of great importance for diagnosis. It is no longer necessary to wait for the end of the starting time of the anode. Furthermore, it is necessary to apply the starting energy to the anode only once during a working period, and it is no longer necessary to apply brakes to very quickly running anodes to avoid wear and noises. In addition, it is possible to make the anodes run faster, so that the speed of rotation can be adapted to the load without having to consider the wear of the bearing. Due to magnetic support, there is no contact of moving parts with the immovable parts, with the exception of one connection through which flows the current of the X-ray tube, such as an axial bearing.

Magnetic bearings are known in the art and are described, for example, in the publication Philips Technische Rundschau, 1960/61, No. 7, pages 252 to 259.

In these bearings, there is used, as the force centering the axle, a repelling force between radially magnetized manent magnets produces all the drawbacks which permanent magnets have in comparison with electromagnets. They age, they lose their residual magnetism at the Curie point, and their residual magnetism even prior to the Curie point is dependent on the temperature of at least up to 300 C reached in the bearings of the rotary anodes. Furthermore, in the case of permanent magnets, the magnetization cannot be steered, at least in the rotor. Permanently magnetized rings considerably increase the actual weight of the rotor and thus the'cost required for its bearing.

On the other hand, electromagnets are difficult to insert into a rotor since they require conduits. A feeding current must be supplied through slides. This again produces friction and wear which must be avoided. This is also one of the difficulties for insertion of a motor into the vacuum space of the tube. One can imagine that magnetic spools built into the rotor could receive current from induction spools also fixed to the rotor. However, field forces resulting from such an energy transmission would act upon the rotor, and this is not desired, since the forces must be taken from the magnetic bearing and they would increase requirements made upon the magnetic bearing.

According to an advantageous construction of the present invention, use is made of the above-described principle, and a structure is used which permits the use of electromagnets without requiring contact of rotary and fixed parts, such as slide contacts. A bearing then consists of two coaxially arranged tubular piles consisting of superposed electromagnetic rings which are alternately opposedly magnetized. One of the piles, consisting of so-called inner magnets, has a small diameter, and is located within the other pile having so-called outer magnets with a large diameter. Direct current flows through ring-shaped field windings of all inner and outer magnets to produce a magnetic field. Alternate opposed direction of magnetizing, which was already accepted as necessary in the above-mentioned publication, results, in that the ring spools are wound in opposite directions. To provide magnetic poles which are precise in space, magnetically conducting lugs directed toward each other are coordinated with the magnetic rings, whereby lugs pertaining to magnets located within the rotor are outwardly located lugs or yokes, while those pertaining to the outer magnets are inwardly directed yokes. The rotor is located without contact in the cylindrical space between the piles of the inner and outer magnets. The rotor consists largely of a non-magnetic material as, for example, copper.

According to one construction of the yoke arrangement, soft magnetic rings are located upon its inner side. They are yokes of inner magnets consisting of the same material, but without touching them. The radial width of the overlapping is such that the poles of the inner magnets extend outwardly, even if there is some excentricity of the rotor relatively to the ringshaped magnets, over the soft magnetic rings, i.e., yokes. of the rotor. Then, the ring-shaped outer magnets can exert repelling forces upon the rotor without it being necessary to apply magnets to the rotor itself. For this. reason, electromagnets can also be used without it being necesary to have slide contacts.

The distance between the inner edges of the yokes of the outer magnets and the outer edges of the yoke rings of the rotor, which can be indicated by the letter 0, should be small so that the force (which is opposed to the eccentricity of the rotor) has the highest possible value, and the axial height ofa magnetic ring should be above three times greater than the distance c, according to statements of best results indicated in the abovestated publication.

When the rotor can receive high voltage relative to the outer magnets, the distance c is fixed by the tension strength of this stretch and is comparatively large to 12 mm.) However, it can also be small, and is determined solely by manufacturing tolerances and the wall strength of the vacuum piston when the outer magnets follow the rotor in their potential.

The yoke rings carried by the rotor overlap the inner magnets without touching them. This overlapping can be provided by segmenting the yoke rings provided in the rotor. Thus, the rotor consists advantageously in the section pertaining to the magnetic bearing out of a non-magnetic outer cylinder and a non-magnetic inner cylinder which is rotationally safely fixed with tight fit in the outer cylinder and which carries yoke rings of the rotor fixed in grooves, possibly soldered. In order to facilitate mounting, the inner cylinder and the yoke rings fixed thereon are cut into two halves in a plane extending through the rotor axis. Thus, the inner cylinder with the yoke rings separates into two equal segments as soon as it is removed from the outer cylinder. These segments can then be placed about the inner magnets and then held together by positioning the outer cylinder. The axially directed attracting forces which each inner magnet exerts upon two yoke rings of the rotor are balanced by themselves due to the arrangement of the rotor yoke rings in pairs in the rotor, provided that the two air gaps between the inner magnet and the two corresponding yoke rings of the rotor are equal in size. If the rotor is shifted axially relative to the inner magnets, even if only to a small extent, then one air gap becomes smaller to the extent of this shifting, and the other one becomes larger to the same extent and the axially operating forces no longer balance themselves in the rotor. Thus, such a premeditated shifting of the rotor can produce a difference in force and if so, the axle of the rotor can always be held in contact with the axial bearing which, at the end of this axle, can have the shape of a needle bearing or a supporting ball, and which constitutes the contact required for the transmission of the anode current. It is advantageous to place the axial bearing at such a distance from the plate of the rotary anode that the axis of the rotary anode intersects the imaginary vertical line upon the focal point surface in the axial bearing. In that case, excentric or top motions of the rotary anode do not produce any focal pivot movements in the first approximation.

In the above-described arrangement, wherein outer magnets act repellently upon yokes of the rotor magnetized by inner magnets and thus upon the rotor, a return movement of the rotor into its central position is 4 provided in that-the repellent force is greatest precisely at the locations of the circumference of the rotor yokes which are closest to the outer magnets due to the eccentricity.

The operation of outer and inner magnets makes possible a damping in the return movement in that high return forces are operative only until a sufficient differential decrease of eccentricity with the time de/dt is reached, so that the rotor will not carry any regulating swingings or only small ones about its central location.

The operation of the magnets is thus dependent upon a signal which is directly connected with the eccentricity of the moment. This signal can be produced by placing two metal cylinders coaxially about the rotor without touching the outer magnets, and the capacity between these two cylinders acts more or less out of time upon a swinging circuit and thus upon the oscillation amplitude of this swinging circuit. The capacity between these two cylinders is then dependent upon the eccentricity, since the electrical fields are formed from one of the two cylinders to the other cylinder substantially over the rotor. Finally, the oscillation amplitude of the resonance circuit can actuate currents in the -field windings of the magnets through an electronic regulating stretch, the characteristic of which is adapted to the top motion conditions of the rotor.

The invention will appear more clearly from the following detailed description when taken in connection with the accompanying drawings, showing, by way of example only, preferred embodiments of the inventive idea.

In the drawings:

FlG. l is a sectional side view of an X-ray tube with rotary anode, suitable for the purposes of the present invention.

FIG. 2 is a transverse section along the line "-11 of FIG. 1.

FIGS. 3 and 4 are sections through two different constructions of magnetic bearings.

FIG. 1 shows a vacuum container 1 having at one end the cathode arrangement 2 and at the other end the anode combination 3. In this case, the actual glow cathode 4 is fixed by a support 5 in an innerly directed tube 6 of the glass container 1. A support 8 is melted into the side of the vacuum container 1 located opposite the tube 6. The bearing Scarries ring-Shaped field windings of electromagnets indicated by numerals 9 to 15. The field windings 9 to 15 are amplified into ring-shaped inner magnets by the yokes 16 to 23 of soft magnetic material as well as the soft magnetic support 8. F urthermore, a plate disc 7 is placed vacuum-tightly in the support 8 by a more or less strong tensioning. This plate holds a point bearing 24, which is slightly shiftable in axial direction. The anode combination 3 is held in axial direction in electrical contact with the member 7 through the point of the carrying spindle 24 located in the bearing 24. The rotor 27 is provided at the upper part of the axle 25, constructed as a spindle close to the anode plate 26. As already stated, the rotor consists of two non-magnetic hollow cylinders stuck one within the other, and carries upon its inner side yokes 28 to 35 of soft magnetic iron. These yokes are magnetically in engagement with yokes 16 to 23 of electromagnets with the field windings 9 to 15. Thus, the yokes 28 to 35 extend further outwardly the fields of the windings 9 to 15. In the illustrated example, the fields of the windings 36 to 42, as locate'dat the outer side of the tube container 1, are arranged precisely in space above the yoke rings 43 to 50 fixed upon the soft magnetic cylinder 71.

These fields repel the yoke rings 28 to 35, and thus act repellently upon the rotor. A stator 52 is arranged in the known manner on the outer side of the container 1 at the rotor end distanced from the rotary anode plate 26 and the actual driving part 51 of the rotor 27. A potential cylinder 72 is provided to keep smallthe space between the yokes 43 to 50 and the vacuum container 1 without causing improperly high electrical field strength at the inner edges of the yokes 43 to 50 (due to high voltage which the anode 3 provides during X-ray photographing relative to ground, thus also relative to outer magnets located at the ground potential).

The functioning of the present invention in the illustrated example takes place in that a current is switched on which is received from the source through conduits 73, 74 and 75 into the switch device 55 and throughthe conduits 56 and 57, and the insulation stretch with secondary rectification 76 is directly transmitted to field windings 9 to and through conduits 77 and 78 to the field windings 36 to 42. Thus, a support of the rotary parts of the anode 3 in the radial direction is produced without contact. This is based on the repelling forces of the magnetic fields emanating from the windings 36 to 42 and acting upon yokes 28 to 35 magnetized by field windings 9 to 15. The capacity between the probe 60 and the potential cylinder 72 is measured from the device 55 through conduits 58 and 59. This capacity is a measure for the immediate eccentricity of the anode combination 3 relatively to the two magnetic ring devices. The device 55 also contains means operating the currents in the conduits 56 and 57, resp. 77 and 78, depending upon the extent of eccentricity and the extent of change of eccentricity in time. The eccentricity or the change of eccentricity in time causes the device 55 to transmit such currents through the conduits 56 and 57 and the conduits 77 and 78 and thus through the field windings of magnets within and outsideof the rotor, that forces are exerted upon the rotor which are opposed to eccentricity, but which act against the eccentricity only to the extent that the rotor will not carry out regulating swingings about its central position.

X-rays are produced in the known manner, on the one hand, by supplying current necessary for sending electrons through the conduits 61 and 62 of the glow cathode 4, and on the other hand, by applying high voltage 65 of a few 10 v. produced by the generator 64 located at the net 73, 74 and 75 between the glow cathode 4 and the conduit 63, which is in galvanic contact with the rotary anode 26. Thus, the electrons emitted by the glow cathode are transmitted by the stator 52 relative to the alternating current of the generator 64 or two-phase alternating current 66 over conduits 67, 68, and 69; they are accelerated by the rotating anode 26, and their kinetic energy is transformed there in a known manner into X-rays which leave the tube as a cone-shaped bundle 70.

FIG. 2 shows the arrangement of the field windings 13 and 40 as well as those of the interengaging yokes and 32 and of the yoke 47 located in one plane in the yoke 32. The wall of the vacuum container 1 is visible between the yoke 32 and the yoke 47. The outer yoke rings 43 to 50 are fixed to the magnetically soft cylinder 71, and the inner yoke rings 16 to 23 are fixed to the soft-magnetic support 8. The spindle moves in the center of thehollow support 8.

FIG. 3 shows the driving part 51' of the rotor 27' and the stator 52 arranged upon the end of the staple of magnets directed to the rotary anode plate 26. Thus, the distance of the staples 79 and 80 from the rotary anode plate 26' is greater than the distance of the rotary anode plate from the magnetic staples according to the construction of FIG. 1. This is of particular advantage when the rotary anode is to be very heavily loaded. Due to the increased space, the passage of heat is also longer, and the magnetic staple 79 as well as the staple will be subjected only to low temperatures. Furthermore, in this construction, the inner staple 79 acts repellently upon the yokes 81 and 82, located at the rotor 27 The yokes 84, 84 and 85 belong to the outer staple 86, which, due to the necessarily smaller air gap between the yokes of the rotor and those of the outer magnets, must lie upon the anode potential. The yokes 83, 84 and 85 extend through the wall 86 of the container l-of the tube. This construction has the advantage that there is an intensive force action of the magnetic field from the staple 79 to the yokes 81 and 82 due to the small gap 87, while, in the construction of FIG. 1, in addition to the gap, the wall of tube container 1 is also located between the magnets and the yokes.

In the construction shown in FIG. 4, the driving member 51" and the stator 52" are arranged in the middle of the length of the carrying spindle 25". Due to this arrangement, the two magnetic staples are divided into two parts; namely, two separate supports are produced. Thus, for inner magnets are produced partial staples 88 and 89, which are held at a distance from each other by a distancing holder 90 corresponding to the length of the driving member 51". The outer magnet is also divided into staples 91 and 92, which are counterparts of the staple parts 88 and 89. In this construction, amechanical separation is provided between the inner staples 88 and 89 and their coordinated rotor yokes 93, 94 and 95, 96, and also between staples 91 and 92 and their yokes 83' to 97 and 98. The space 87 is the same as the space 87 of FIG. 3. Only the wall 99 of the container 1" consists of glass and is not penetrated by the yokes 83 to 85, 97 and 98, but on the inner side of the wall 94, there is a holding coating 100 which holds the yokes 83 to 85, 97 and 98. This construction, in addition to deviating from the constructive structure of the yokes and the magnetic staples, has the advantage that the magnetic supports can have the greatest possible distance from each other in the manner usual for ball bearings at a predetermined length of the axle 25". In this construction, a stable support is produced while accepting heating in staple 88 greater than in construction of FIG. 3.

The length of the axle 25 (FIG. 1) is so selected that the imaginary vertical line 101 upon the focal point path 102 of the anode 26 cuts in the contact point of theaxial bearing 24. This can be the supporting point of the pointed end of the axle 25 constructed as a spindle. A similar axial support is provided by a ball 104 (FIG. 4) located between the end of the axle 25" having a flat or concave shape and the counter bearing 105, namely, the inner wall of the connecting member 106.

What is claimed is:

1. A process for operating rotary anodes of X-ray tubes, comprising magnetically supporting the anode free from contact except for a support transmitting the X-ray current and rotating the anode continuously during the entire period in which there is a possibility that an X-ray photograph will be taken.

2. The process in accordance with claim l'wherein the magnetic support is radial and the current transmitting support is vertical.

3. A rotary anode X-ray tube provided with a mechanical bearing means for giving mechanical support to the rotary anode when it is rotating in its operative disposition and serving to conduct tube current passing by way of the anode, and magnetic bearing means adapted to give additional support to the anode magnetically when it is so rotating thereby enabling the anode to remain in the said operative disposition, while rotating, without further mechanical support, said magnetic bearing means being arranged to give the anode radial support relative to its rotary axis. and said mechanical bearing means being arranged to give the anode axial support.

4. An X-ray tube as claimed in claim 3 wherein said mechanical bearing is adapted to give the anode vertical support.

5. An X-ray tube as claimed-in claim 3 wherein the said magnetic bearing means comprises afirst stack of aligned annular electromagnet coils which extend,'lo'ngitudinally of the rotary axis of the anode, along and within a second stack of such coils, said first stack being located radially inwards of a rotary part of the anode that is surrounded by the said second stack in an internal space bounded by the said rotary part, alternate coils in each of the stacks being adapted to produce mutually opposed magnetic fields when energized in a predetermined manner, and the said rotary part having magnetically conductive portions arranged to extend. with clearance, between said first and second stacks so that magnetic forces exerted on the said rotary parts by the stacks when the coils thereof are energized hold the anode in its operative disposition when rotating.

6. An X-ray tube as claimed in claim 5 wherein the 7 said rotary part is a hollow cylindrical part arranged concentrically about the rotary axis of the anode.

7. An X-ray tube as claimed in claim 5 wherein the said magnetically conductive portions comprise a plurality of annular members which extend around the said rotary axis and are aligned longitudinally thereof.

8 An X-ray tube as claimed in claim 5 wherein the said first and second stacks are arranged concentrically around the said rotary axis.

9. An X-ray tube as claimed in claim 5 wherein the said magnetically conductive portions are arranged so as to rotate, when'the anode is rotating in its operative disposition, freely past respective further magnetically conductive portions that are mounted radially inwards of the said rotary part and serve as magnetic yokes for the said first stack.

l0. An X-ray tube as claimed in claim 9 wherein the said further magnetically conductive portions comprise a plurality of annular members which extend along the said rotary axis and are aligned longitudinally thereof.

:11. An X-ray tube as claimed in claim 5 wherein the said magnetically conductive portions of the said rotary part are arranged so as to rotate, when the anode is rotating in its operative disposition, freely past respective further magnetically conductive portions that are mounted radially outwards of the said rotary part and serve as magnetic yokes for the said second stack.

12. An X-ray tube as claimed in claim S Wherein the anode has arranged axially between an anode plate thereof and that end of the said stacks nearer to'the anode plate a drive part with which a stator winding extending freely aroundthe drive part is adapted to cooperate to cause rotation of the anode when the stator winding is energized in a predetermined manner.

13. An X-ray tube as claimed in claim 5 wherein said stacks are divided into two parts which are separated from one. another, axially of the anode, by a drive part thereof which is surrounded by a stator winding which is adapted to cooperate with the drive part so as to cause rotation of the anode when the stator winding is energized in a predetermined manner.

14. Apparatus as claimed in claim 3 wherein said mechanical bearing is arranged to support theanode axially at a point situated where the rotary axis of the anode intersects an imaginary perpendicular drawn from an annular target surface which the anode provides, extending concentrically about the said .rotary axis, for an incident electron beam.

15. An X-ray tube as claimed in claim 3, said tube being provided with sensing means arranged to detect eccentricity of rotation of the said anode and provide an electrical signal which is a measure of such eccencounteract such eccentricity by damping. 

1. A process for operating rotary anodes of X-ray tubes, comprising magnetically supporting the anode free from contact except for a support transmitting the X-ray current and rotating the anode continuously during the entire period in which there is a possibility that an X-ray photograph will be taken.
 2. The process in accordance with claim 1 wherein the magnetic support is radial and the current transmitting support is vertical.
 3. A rotary anode X-ray tube provided with a mechanical bearing means for giving mechanical support to the rotary anode when it is rotating in its operative disposition and serving to conduct tube current passing by way of the anode, and magnetic bearing means adapted to give additional support to the anode magnetically when it is so rotating thereby enabling the anode to remain in the said operative disposition, while rotating, without further mechanical support, said magnetic bearing means being arranged to give the anode radial support relative to its rotary axis, and said mechanical bearing means being arranged to give the anode axial support.
 4. An X-ray tube as claimed in claim 3 wherein said mechanical bearing is adapted to give the anode vertical support.
 5. An X-ray tube as claimed in claim 3 wherein the said magnetic bearing means comprises a first stack of aligned annular electromagnet coils which extend, longitudinally of the rotary axis of the anode, along and within a second stack of such coils, said first stack being located radially inwards of a rotary part of the anode that is surrounded by the said second stack in an internal space bounded by the said rotary part, alternate coils in each of the stacks being adapted to produce mutually opposed magnetic fields when energized in a predetermined manner, and the said rotary part having magnetically conductive portions arranged to extend, with clearancE, between said first and second stacks so that magnetic forces exerted on the said rotary parts by the stacks when the coils thereof are energized hold the anode in its operative disposition when rotating.
 6. An X-ray tube as claimed in claim 5 wherein the said rotary part is a hollow cylindrical part arranged concentrically about the rotary axis of the anode.
 7. An X-ray tube as claimed in claim 5 wherein the said magnetically conductive portions comprise a plurality of annular members which extend around the said rotary axis and are aligned longitudinally thereof.
 8. An X-ray tube as claimed in claim 5 wherein the said first and second stacks are arranged concentrically around the said rotary axis.
 9. An X-ray tube as claimed in claim 5 wherein the said magnetically conductive portions are arranged so as to rotate, when the anode is rotating in its operative disposition, freely past respective further magnetically conductive portions that are mounted radially inwards of the said rotary part and serve as magnetic yokes for the said first stack.
 10. An X-ray tube as claimed in claim 9 wherein the said further magnetically conductive portions comprise a plurality of annular members which extend along the said rotary axis and are aligned longitudinally thereof.
 11. An X-ray tube as claimed in claim 5 wherein the said magnetically conductive portions of the said rotary part are arranged so as to rotate, when the anode is rotating in its operative disposition, freely past respective further magnetically conductive portions that are mounted radially outwards of the said rotary part and serve as magnetic yokes for the said second stack.
 12. An X-ray tube as claimed in claim 5 wherein the anode has arranged axially between an anode plate thereof and that end of the said stacks nearer to the anode plate a drive part with which a stator winding extending freely around the drive part is adapted to cooperate to cause rotation of the anode when the stator winding is energized in a predetermined manner.
 13. An X-ray tube as claimed in claim 5 wherein said stacks are divided into two parts which are separated from one another, axially of the anode, by a drive part thereof which is surrounded by a stator winding which is adapted to cooperate with the drive part so as to cause rotation of the anode when the stator winding is energized in a predetermined manner.
 14. Apparatus as claimed in claim 3 wherein said mechanical bearing is arranged to support the anode axially at a point situated where the rotary axis of the anode intersects an imaginary perpendicular drawn from an annular target surface which the anode provides, extending concentrically about the said rotary axis, for an incident electron beam.
 15. An X-ray tube as claimed in claim 3, said tube being provided with sensing means arranged to detect eccentricity of rotation of the said anode and provide an electrical signal which is a measure of such eccentricity, and with energization control means arranged to control the supply of energization current to the electromagnet coils of the first and second stacks in dependence upon the said electrical signal, thereby to counteract such eccentricity by damping. 